1. Field of the Invention
This invention relates to fiber optic communications, and more particularly to a system and method for rapidly switching among a number of optical rods in a multirod cable for optical connection with a communications channel.
2. Description of the Prior Art
In recent years optical technology has emerged as a primary vehicle for communications systems, such as telephone networks, in which rapid switching must be accomplished among large numbers of different lines. Optical communications cables typically have a relatively large number of separate rods, each of which is capable of carrying a discrete optical signal. (The term "rod" is used herein to indicate a discrete signal line, and can consist of either a single optical fiber or a bundle of fibers. The term "optical" is used in the broad sense to refer not only to visible light, but also to adjacent areas of the electromagnetic spectrum.) It is often necessary to optically interconnect two or more such cables, and to be able to rapidly switch the interconnection among the various rods of each cable. For example, time-based multiplexing techniques in common use enable a large number of different conversations to be transmitted over a single communications channel by obtaining short samples of each conversation in sequence, and interleaving the samples on the communications channel. The discrete samples are separated at the opposite end of the channel and used to reconstruct the original conversations.
In order to couple a cable having a number of optical rods with a communications channel, a lens has typically been positioned adjacent the end of the cable to direct light emitted from the ends of selected rods onto the channel for transmission to a desired location. Switching among the different rods in the cable has been accomplished by holding the lens stationary and physically indexing the cable so that the desired rod is moved into alignment with the focal axis of the lens. As rapid switching such as multiplexing progresses, the cable is repeatedly indexed so that each desired rod is sequentially brought into alignment with the lens.
This method has resulted in some serious problems in the ability to obtain accurate signals for transmission, and to rapidly switch among the various rods in the cable. It is difficult to maintain mechanical alignment between the fixed lens and movable cable, and alignments can be adversely effected by variables such as thermal expansion or shrinkage. Such systems are generally difficult to install, and do not exhibit a particularly long working life. In high vibration environments such as aircraft, the optical components are subject to serious misalignment from motor vibration.
It is also difficult to achieve high speed switching with prior optical systems. Whereas the optical rods are typically 10-20 microns in diameter, a much larger spacing has been required between the rods to accommodate the necessary mechanical motion. The spacing between rods is typically in the order of 0.1 inch, or approximately 3,000 microns. This results in the need for substantial displacements of the cable during switching, and an accompanying loss of switching speed.